To guarantee high forgery-proofness, bank notes are inter alia provided with magnetic properties. During automated bank note testing in bank note processing apparatus, bank notes are therefore also examined for their magnetic properties to distinguish counterfeits or suspected counterfeits from authentic bank notes.
Herein the examination of the magnetic properties of bank notes is usually effected using inductive measuring heads, Hall elements or magnetoresistive elements, such as field plates or thin permalloy layers.
In addition, it is known to examine the magnetic properties of bank notes using magneto-optical layers. A suitable device is for example known from the German laid-open publication DE 197 18 122 A1. Herein a magneto-optical reflector layer with a high magnetic Kerr effect is illuminated with polarized light and the reflected light is detected after passing through a polarization filter. If a bank note to be examined is brought close behind the reflector layer, the magnetic leakage flux of the magnetic areas of the bank note influences the optical behavior of the reflector layer, thereby changing the polarization direction of the detected light. From the measured change of polarization one can then infer the magnetic properties of the sheet material. Compared to the frequent use of inductive measuring heads, the use of magneto-optical layers has the advantage that they allow higher spatial resolution and the measurement of magnetic flux is independent of the speed of the bank note relative to the measuring system. In addition, the use of magneto-optical layers allows the use of an imaging method for the visualization of the magnetic patterns incorporated in the bank note.
The examination of the magnetic properties of bank notes by machine involves in particular the problem that very small magnetic flux densities must be detected to be able to guarantee a sufficiently precise and reliable verification of authenticity. This is because, firstly, the leakage flux caused by the individual magnetic areas of the bank notes is very small and, secondly, the typical distances between bank note and magneto-optical layer cannot be reduced at will due to the high transport speed required in bank note processing apparatus, as this would otherwise lead to elevated wear of the bank notes to be checked as well as individual sensor components and in addition result in an elevated risk of jams.
From WO 02/052498 A2 a device and a method for investigating the magnetic properties of objects are known, in which magneto-optical layers with regularly arranged magnetic domains are used. Therein light generated by a light source incident upon the magneto-optical layer is diffracted by the regularly arranged magnetic domains. The light which is diffracted and transmitted or reflected by the layer is received by a sensor. If an object, in particular a sheet, with magnetic areas is disposed close to the magneto-optical layer, the magnetic areas of the sheet influence the optical properties of the magnetic layer, wherein the distances and/or widths of the regularly arranged magnetic domains vary in accordance with the direction and intensity of the magnetic field of the sheet acting on the magneto-optical layer. The detected intensity and/or position of the diffracted light changes correspondingly depending on the magnetic properties of the sheet, so that the magnetic properties of the sheet can be inferred therefrom.
The known device and the method for examining magnetic properties of objects by means of magneto-optical layers with regularly arranged magnetic domains have the advantage that the used magneto-optical layers with domains have a high sensitivity, for which reason they are suitable for the detection of very small changes in the density of the magnetic flux. However, the spatial resolution is limited by the size of the magnetic domains.
From WO 02/052512 A2 a device and a method for examining magnetic properties of objects are known, in which a magneto-optical layer is used which is arranged as a so-called in-plane layer. Such in-plane layers do not have any magnetic domains, or rather one single magnetic domain lies in the layer itself and extends parallel thereto. Such magneto-optical layers have the advantage that they allow practically any spatial resolution. However, the sensitivity of the in-plane layers for changes of the density of the magnetic flux is substantially lower than that of the magneto-optical layers with magnetic domains. Therefore in the known method and the device the change, i.e. the rotation, of the polarization direction of the light coupled into the magneto-optical layer is increased by increasing the optical path length of the light passing through the magneto-optical layer. For this purpose the light source and the magneto-optical layer are so disposed that the direction of propagation of the light coupled into the layer extends substantially parallel to a base surface of the magneto-optical layer.
However, the known device and the method for examining magnetic properties of objects by means of magneto-optical in-plane layers have the shortcoming that the production of such in-plane layers is complex and therefore expensive.
Further problems arise if in addition to objects to be examined with strictly predetermined magnetic properties also such objects are examined which have magnetic properties that are undetermined, so that they defy examination, or rather are not accessible to examination without difficulty.